1. Field of the Invention
The present invention relates to a method for continuously plating a steel strip with molten metal, and more particularly to a method for controlling coating weight on a steel strip.
2. Description of the Related Arts
Sink rolls, pinch rolls and wiping nozzles are used in the prior art methods for continuously plating a steel strip with molten metal. The sink rolls are positioned in a coating bath.
A steel strip is moved by the sink rolls and drawn out above the coating bath. The pinch rolls are used for pressing the steel strip down and straightening a bowed steel strip. An excess plating on the steel strip drawn out above the coating bath is removed by jetting a gas from wiping nozzles, whereby a uniform thickness of plating can be obtained.
In the prior art methods, when a moving speed of a steel strip is increased to enhance the productivity of steel strip, coating weight on the steel strip is increased. It is necessary to increase a pressure of gas from the wiping nozzles to lower the coating weight on the steel strip. When the pressure of the gas from the wiping nozzles is enhanced, a gas jet strikes against the steel strip at a high speed and scrapes away an excess molten metal downward. The gas jet having struck against the steel strip generates a flow of associated gas. Parts of molten metal are converted to a splash by the flow of associated gas. The splash having attached to the wiping nozzles clogs the nozzles. As a result, gas cannot be jetted from the wiping nozzles uniformly on the steel strip and the coating weight on the steel strip cannot be uniform.
When the coating weight on the steel strip is not uniform, the appearance of the steel strip is not good. An unequal coating weight on the steel strip can also make an unequal coiling of the steel strip and causes an alloying of the plating generated after plating the steel strip to be unequal. Parts of the splash attach to the steel strip again, which becomes a cause of defects of the steel strip. The increase of the amount of gas from the wiping nozzles gives rise to the increase of cost and generates a noise.
Various methods for removing an excess molten metal on a steel strip in response to an increase of a plating speed have been reported.
Japanese Examined Patent Publication No. 7444/69 discloses a method wherein an eddy current is generated in a steel strip by applying a high-frequency magnetic field to the steel strip and a molten metal is removed by Lorentz force generated by the eddy current. In this case, a wiping of molten metal by means of gas is used together with the application of the high-frequency magnetic field to the steel strip.
According to a method disclosed in Japanese Examined Patent Publication No. 227158/86, a stationary electric current is flowed in a steel strip and Lorentz force generated by .the stationary electric current with a static magnetic field removes an excess molten metal on the steel strip. After the excess molten metal has been removed from the steel strip, the steel strip is wiped by gas.
According to a method disclosed in Japanese Patent Application Laid Open No. 204363/86, a static magnetic field is generated on a surface of a steel strip in an outward direction from the surface of the steel strip, and an excess molten metal is removed by Lorentz force generated by an induced current generated in the molten metal by movement of the steel strip and a static magnetic field. Further, the steel strip is wiped by gas.
Japanese Patent Applications Laid Open No. 266560/86 and No. 103353/87 disclose a method wherein an excess molten metal is moved downwardly by a moving magnetic field generated in a downward direction from the steel strip and the steel strip is wiped by gas.
In those conventional methods, however, when the magnetic field is caused to act on a ferromagnetic body such as steel strip, the steel strip is drawn toward a stronger magnetic field. In consequence, a system as a whole becomes unstable, and it is difficult to appropriately control the system. To avoid this difficulty, it is necessary to space out a magnetic field generator and the steel strip. When the magnetic field generator and the steel strip are spaced out, the effect of the magnetic field is limited, which does not produce an effect as an object.
Vibration of a steel strip and bowing of a steel strip in a width direction thereof can be the causes that coating weight on the steel strip is not uniform. Any of the above-described conventional methods has no effect on damping of vibration and straightening of bowing of the steel strip. It is described in Japanese Examined Patent Publication No. 7444/69 that a steel strip passing between coils is centered between the-coils by a magnetic repulsive force. However, since a magnetic attracting force strongly acts on the steel strip as a ferromagnetic body when a high-frequency magnetic field is simply applied to the steel strip, the steel strip passing between the coils is attracted toward the coils. As a result, an unstable vibration of the steel strip is generated. Therefore, any damping of vibration of the steel strip cannot be expected in accordance with this method.